Coming from the Serious Games business myself, I wholeheartedly agree that XNA is a great tool for that industry.
Serious Games suffer from the problem that to be really useful, they need to be as realistic as possible. They have difficulties because in order to make a realistic simulation, you either need an advanced game engine (such as HalfLife2) or need to develop your own. Advanced engines are generally out, because their liscenses and prices are optimized for wide distribution of games rather than the market for serious games, which is often school districts, law enforcement, or other small government agencies with small budgets. Developing your own engine, however, costs time and money, and advanced developers and artists that don't come cheap.
Enter XNA, which allows rapid development in a managed library, along with a solid library of easy to use game essentials. The liscense is good, allowing you to create a windows-based commercial serious game for a fraction of the cost. The number of programmers needed is reduced (though art requirements remain the same), and the amount of development that can be done in short time increased. In all, XNA is perfect for the serious games industry.